Electroplating additive for improved reliability

ABSTRACT

Described are methods of and compositions for electrodepositing copper or other metals onto interconnects of a semiconductor substrate from an electroplating composition containing at least one nitrogen-containing additive. The nitrogen-containing additive has a molecular weight of between 10 and 1000, a concentration of between 5.0 and 10.0 milligrams per liter of the electroplating composition. The methods and compositions result in electroplated copper interconnects that have smooth surfaces that are relatively free of pits and humps.

BACKGROUND OF THE INVENTION

In an effort to make semiconductor transistors faster and more reliable, copper interconnects have become the favored material over conventional aluminum interconnects as described in Gau et al., Copper electroplating for future ultralarge scale integration interconnection, 18(2) J. VAC. Sci. TECH. A 656-60 (2000). From a materials point of view, copper is a better conductor than aluminum due to its lower electrical resistance. Copper is also less susceptible to electromigration, a phenomenon that occurs when a constant dense flow of current dislocates atomic structures and creates voids in the metal interconnects. Furthermore, copper has higher thermal conductivity than aluminum, which translates into faster heat dissipation and lower energy consumption. As a result, electroplated copper is currently used by various major semiconductor manufacturers as an interconnect material.

In general, copper electroplating involves an acidic plating composition containing (1) a dissolved copper salt such as copper sulfate, (2) an acidic electrolyte such as sulfuric acid or hydrochloric acid in an amount sufficient to impart conductivity on the bath, and (3) additives such as surfactants, brighteners, levelers and suppressants to enhance the effectiveness and quality of plating. See generally U.S. Pat. Nos. 5,174,886; 5,068,013; 5,051,154; and 3,876,513 for discussions on electroplating compositions.

SUMMARY OF THE INVENTION

Disclosed are methods of electrodepositing copper or other metals onto interconnects of a semiconductor substrate from an electroplating composition containing at least one nitrogen-containing additive. The nitrogen-containing additive has a molecular weight of between 10 and 1,000, and more specifically may have a molecular weight of between about 31 and 500; the additive may also have a concentration of between 5.0 and 10.0 milligrams per liter of the electroplating composition, with the operational plating temperature of the electroplating composition being less than approximately 50° C., and in certain embodiments from about 10° C. to 35° C. In one embodiment, the nitrogen-containing additives are primary, secondary, and tertiary amines. In another embodiment, the nitrogen-containing additives are aliphatic, cyclo-aliphatic, aromatic, and heterocyclic amines. In yet another embodiment, the nitrogen-containing additives are aliphatic, cyclo-aliphatic, aromatic, and heterocyclic quaternary ammonium compounds. In a further embodiment, the nitrogen-containing additives are imidazole, pyridine, and their derivatives. The nitrogen-containing additives may specifically comprise C₃H₄N₂ or C₅H₅N. The nitrogen-containing additive may have an asymmetric charge density distribution with respect to a core chemical structure in comparison with amines.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the principles disclosed herein, and the advantages thereof, embodiments are illustrated by way of example in the following figures in which like reference numbers indicate similar parts, and in which:

FIGS. 1A-1E are schematic cross-sectional diagrams illustrating the results of progressive stages of copper electroplating; and

FIG. 2 compares the lifetime and reliability of semiconductor devices utilizing conventional processes and those utilizing the disclosed embodiments.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Initial reference is made to FIGS. 1A-1E which shows a series of schematic cross-sectional diagrams illustrating the results of progressive stages in electroplating copper. In FIG. 1A, an interconnect opening such as a via or a trench 100 has been defined within a dielectric layer 102 over a conductive layer 104 and on top of a semiconductor substrate 106 using known methods and techniques. The dielectric layer 102 comprises silicon nitride or silicon oxide or dielectric film containing carbon atoms while the conductive layer 104 comprises various levels of the integrated circuit such as interlevel metal dielectric, gate electrodes, interlevel dielectric, isolation regions, capacitors and other features. The trench 100 may also be defined within the dielectric layer 102 directly on top of the semiconductor substrate 106 without the conductive layer 104 in between them. Furthermore, the trench 100 may also be defined within the semiconductor substrate 106 itself, although electroplating is most often done on some interconnect level above the semiconductor substrate 106. The trench 100 has an aspect ratio defined as the ratio of the depth 110 of the trench 100 relative to the width 108 of the trench 100. For example, if the trench 100 has a depth 110 of 1 micron and a width 108 of 0.1 micron, then the trench 100 has an aspect ratio of 10:1 (1 micron deep: 0.1 micron wide). Preferably, the aspect ratio within the present embodiments is at least 4:1. However, the present embodiments may also be used for lower aspect ratio of 2:1 or 3:1.

A barrier adhesion layer 112 is subsequently blanket deposited as illustrated in FIG. 1B. The barrier adhesion layer 112 works as a diffusion barrier by preventing the electroplated copper from spiking or diffusing into the conductive layer 104 or the semiconductor substrate 106. This barrier adhesion layer 112 comprises titanium nitride, tantalum nitride, or nitrided titanium-tungsten, or other refractory metal alloy containing cobalt film deposited by sputtering or chemical vapor deposition or atomic layer deposition techniques. The barrier adhesion layer 112 also helps to improve the adhesion of the electroplated copper. A copper seed layer 114, acting as the plating current conductor and vital to the initiation of the electroplating process, is then blanket deposited as illustrated in FIG. 1C. The copper seed layer 114 is deposited by methods and techniques similar to that of the barrier adhesion layer 112 and comprises mixtures of thin layers of titanium, chromium and copper.

The device as illustrated in FIG. 1C subsequently undergoes the electroplating process resulting in formation of the electroplated copper film 116 as illustrated in FIG. 1D. The electroplating process generally comprises: (1) immersing the wafer into the electroplating composition within the electroplating tool; (2) applying a plating bias to the wafer and plating the metal onto the wafer; and (3) removing the wafer from the electroplating composition and the electroplating tool. In addition to electroplating metal conductive layers of copper, the plating materials may also comprise aluminum, silver, or gold to form electroplated conductive layers of these metals.

As a result of the electroplating process, artifacts or defects will be generated including voids 119 within the metal interconnect and elevated hump formations 118. Voids 119 are created when electroplated metal bridges together within the interconnect openings as a result of demanding sub-micron geometries while hump formations 118 are protrusions above the electroplated surface that can cause pit defects in the metal interconnects during subsequent chemical mechanical polish processes. Although voids 119 usually do not form with electroplating, as geometric features get smaller they can become one of the challenges. Both voids 119 and hump formations 118 can be controlled by the chemical composition of additives within the electroplating composition and by the process conditions under which it is formed. Additives such as levelers or leveling agent provide a leveling effect by giving the electroplated copper a smooth surface when it forms faster and thicker deposits in the interconnect openings and slower and thinner deposits around the peaks. As a result, an effective leveler can solve the poor gap filling void 119 problems and reduces the non-uniformity within the die by minimizing hump formations 118. Consequently, an effective leveling agent increases the lifetime and performance of the semiconductor device by improving its reliability, as fewer voids and increased uniformity translates into lower susceptibility of electromigration and resulting device failure. Subsequently, the wafer as illustrated in FIG. 1D is subjected to a chemical mechanical polish to planarize or smooth the protrusions and another conductive layer 120 is then deposited over the polished electroplated copper 116 as illustrated in FIG. 1E. The conductive layer 120 may not be necessary if the electroplated copper is to serve as the final electrical metal interconnect in the semiconductor device.

Conventional leveling agents use cationic polymers with large molecular weights (10,000-50,000) to achieve the leveling effect of electroplated copper without overplating the features and degrading the gap fill performance. However, due to their large molecular weights and strong cationic properties, traditional leveling agents incorporate themselves into the copper film resulting in metal lines with high levels of impurity. Consequently, this impurity translates into pit defects in the metal lines thereby causing performance degradation of the semiconductor device. The impurity may be an organic material comprising chlorine, carbon, nitrogen, sulfur, or oxygen elements.

Nitrogen-containing amine compounds with low molecular weights make effective leveling agents because of their reduced rates of adsorption and incorporation. With a molecular weight of between 10 and 1,000 (50 to 500 preferred), the level of impurity within the copper interconnects is substantially reduced because of the increased mobility of low molecular weight amines. The types of amine compounds include but are not limited to substituted and unsubstituted primary, secondary, and tertiary, and can further be sub-classified to contain aliphatic, cyclo-aliphatic, aromatic, and heterocyclic structures. In addition, the concentration of the amine compounds should be between about 5.0 and 10.0 milligrams per liter of the electroplating composition with the temperature of the electroplating composition being less than approximately 50° C., and in specific embodiments at between 10° C. and about 35° C. The combination of a relatively high concentration of nitrogen-containing additive and low plating temperature translates into increased mobility and functionality of the additives while restricting the incorporation of larger and bulkier impurities that might exist within the electroplating composition.

Like amine compounds, nitrogen-containing additives such as quaternary ammonium compounds also make effective leveling agents because they contain a protonated charge to inhibit copper impurity adsorption or incorporation, C₃H₄N₂ for example. A protonated charge is when the atom has given up at least one electron thereby giving the overall compound a net positive charge of at least one proton. The protonated charge produces an asymmetric charge density distribution with respect to a core chemical structure, thereby inhibiting impurity adsorption and incorporation. An asymmetric charge density distribution means that the positive charge is not equally distributed throughout the ammonium compound, which translates into one end of the compound inhibiting any bulk impurity while the other end expediting the electroplating process. The types of quaternary ammonium compounds include but are not limited to substituted and unsubstituted aliphatic, cyclo-aliphatic, aromatic, and heterocyclic quaternary ammonium compounds. In addition, other nitrogen-containing additives such as imidazole, pyridine, and their derivatives are also effective leveling agents because they also have an asymmetric charge density distribution with a closed chemical formula so as to provide good gap filling characteristics and substantially pits-free electroplated copper surfaces upon subsequent chemical mechanical polish processes.

FIG. 2 illustrates the increased lifetime and improved reliability of the semiconductor devices utilizing the disclosed embodiments. A standard reliability graph 122 has stress time 124 with units of hours plotted on the x-axis and failure probability 126 with units of percentage plotted on the y-axis. Stress time 124 is a measure of how long the device was stressed at some fixed test bias condition before it failed, while failure probability 126 is an indication of what percentage of devices still survive at that particular stress time 124. For example, if 100 devices were utilized in the reliability test and 10 devices failed after biasing for 1 hour and 50 devices failed after biasing for 5 hours, then the failure probability 126 at 10% would have a stress time 124 of 1 hour while the failure probability 126 at 50% would have a stress time 124 of 5 hours.

A comparison of electroplated copper process utilizing conventional leveler 128 and an electroplated copper process utilizing the disclosed embodiments 130 showed that at nearly every failure probability 126, the disclosed embodiment process 130 has longer stress time than the conventional process 128, which indicates that the electroplated interconnects of the disclosed embodiments have higher reliability than those utilizing conventional levelers. The conventional leveler 128 utilizing additives with large molecular weights (10,000-50,000) produces average humps of about 3,500 Å while the electroplated copper utilizing the disclosed embodiment 130 additives with low molecular weights (100-200) produced humps of about 1,500 Å and 800 Å. In addition, as illustrated in the data summary table 132, the time to failure at the 50% probability (T₅₀) 134, which is the standard specification reported for semiconductor device reliability, showed a more than twofold increase in terms of stress time between the disclosed embodiment 130 at 58.90 hours versus that of the conventional process 128 at 22.07 hours.

Although the above measurements of performance of inventive embodiments show their features and advantages relative to certain prior art implementations, these examples and their performance specifications should not be construed in any way as limitations on the invention or inventions disclosed. The scope of coverage for any patent that issues shall be defined by the claims that any such patent contains. It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description, and all changes that come within the meaning and ranges of equivalents thereof are intended to be embraced therein.

Additionally, the section headings herein are provided for consistency with the suggestions under 37 C.F.R. § 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” the claims should not be limited by the language chosen under this heading to describe the so-called technical field. Further, a description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the “Summary of the Invention” to be considered as a characterization of the invention(s) set forth in the claims found herein. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty claimed in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims associated with this disclosure, and the claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of the claims shall be considered on their own merits in light of the specification, but should not be constrained by the headings set forth herein. 

1. A method for electrodepositing a metal onto at least some conductors of a semiconductor device, the method comprising: (a) providing an electroplating composition containing the metal and at least one nitrogen-containing additive, the nitrogen-containing additive having a concentration of between about 5.0 and 10.0 milligrams per liter of the electroplating composition; (b) immersing the semiconductor device, including the at least some conductors, in the electroplating composition or in a solution comprising the electroplating composition; and (c) electroplating the at least some conductors of the semiconductor device.
 2. A method according to claim 1, wherein the at least one nitrogen-containing additive is an amine compound.
 3. A method according to claim 2, wherein the amine compound is selected from the group consisting of primary amines, secondary amines, tertiary amines, aliphatic amines, cyclo-aliphatic amines, aromatic amines, heterocyclic amines, and combinations thereof.
 4. A method according to claim 1, wherein the at least one nitrogen-containing additive is a quaternary ammonium compound.
 5. A method according to claim 1, wherein the at least one nitrogen-containing additive comprises C₃H₄N₂ or C₅H₅N.
 6. A method according to claim 1, wherein the at least one nitrogen-containing additive comprises a material that is selected from the group consisting of imidazole, pyridine, and their derivatives.
 7. A method for electrodepositing a metal onto at least some conductors of a semiconductor device, the method comprising providing an electroplating composition containing at least one nitrogen-containing additive having a molecular weight of between about 31 and about 500, wherein the nitrogen-containing additive comprises cyclic molecular chemical structure.
 8. A method according to claim 7, wherein the at least one nitrogen-containing additive is an amine compound.
 9. A method according to claim 8, wherein the amine compound is selected from the group consisting of cyclo-aliphatic amines, aromatic amines, and heterocyclic amines.
 10. A method according to claim 7, wherein the at least one nitrogen-containing additive is a quaternary ammonium compound.
 11. A method according to claim 7, wherein the at least one nitrogen-containing additive comprises C₃H₄N₂ or C₅H₅N.
 12. A method according to claim 7, wherein the at least one nitrogen-containing additive is selected from the group consisting of imidazole, pyridine, and their derivatives.
 13. An electroplating material for electrodepositing a conductive layer on a semiconductor substrate, the electroplating material containing at least one nitrogen-containing additive having a concentration of between about 5.0 and 10.0 milligrams per liter of the electroplating material, wherein the at least one nitrogen-containing additive comprises cyclic molecular chemical structure.
 14. An electroplating material according to claim 13, wherein the at least one nitrogen-containing additive is an amine compound.
 15. An electroplating material according to claim 14, wherein the amine compound is selected from the group consisting of cyclo-aliphatic amines, aromatic amines, and heterocyclic amines.
 16. An electroplating material according to claim 13, wherein the at least one nitrogen-containing additive is a quaternary ammonium compound.
 17. An electroplating material according to claim 13, wherein the at least one nitrogen-containing additive contains a protonated charge to inhibit organic impurity adsorption or incorporation.
 18. An electroplating material according to claim 13, wherein the at least one nitrogen-containing additive comprises C₃H₄N₂ or C₅H₅N.
 19. An electroplating material according to claim 13, wherein the at least one nitrogen-containing additive is selected from a group consisting of imidazole, pyridine, and their derivatives.
 20. An electroplating material according to claim 13, wherein the conductive layer is selected from the group consisting of copper, aluminum, silver, and gold. 